Rechargeable batteries are widely used for powering small portable electronic devices. Lithium ion batteries offer a high energy density and high operating voltage, together with a reasonable life. Other rechargeable batteries commonly used are lead batteries and nickel-metal-hydride batteries.
Sealed rechargeable battery cells can pose an explosion hazard when internal gas pressure rises uncontrollably, through over-charge or over-discharge. U.S. Pat. No. 4,025,696 describes a cell with an internal overpressure safety switch responsive to deflection of the walls of the cell for breaking an electrical contact. The cell has a casing with cylindrical side walls and incorporates a Belleville spring element adjacent the bottom of the casing which is moved from a first to a second stable position by outward bulging of the bottom of the casing. However these switches take up space within each cell in a multi-cell battery, and are expensive.
Common over-pressure relief devices include vents or seals which are opened or ruptured under excess pressure. The venting or rupture are irreversible and once the seal of the cell is broken the corrosive and potentially harmful electrolyte can escape.
As an alternative, or further safeguard, rechargeable batteries are often provided with an integrated protection circuit which stops the charging of the battery due to avoid overcharging. The circuit senses battery temperature and/or pressure and cuts off current flow through the cell. These circuits are also expensive and suffer from a lack of positive action. It is an object of the present invention to overcome or substantially ameliorate the above disadvantages or more generally to provide an improved rechargeable battery.